Treatment of cold start engine exhaust

ABSTRACT

The present invention relates to a process for reducing cold start emissions in an exhaust gas stream (such as from an internal combustion engine) by contacting the exhaust stream with a combination of molecular sieves comprising (1) a small pore crystalline molecular sieve or mixture of molecular sieves having pores no larger than 8 membered rings selected from the group consisting of SSZ-13, SSZ-16, SSZ-36, SSZ-39, SSZ-50, SSZ-52 and SSZ-73 molecular sieve and having a mole ratio at least 10 of (a) an oxide of a first tetravalent element to (b) an oxide of a trivalent element, pentavalent element, second tetravalent element which is different from said first tetravalent element or mixture thereof and (2) a medium-large pore crystalline molecular sieve having pores at least as large as 10 membered rings selected from the group consisting of SSZ-26, SSZ-33, SSZ-64, zeolite Beta, CIT-1, CIT-6 and ITQ-4 and having a mole ratio of at least 10 of (a) an oxide of a first tetravalent element to (b) an oxide of a trivalent element, pentavalent element, second tetravalent element which is different from said first tetravalent element or mixture thereof.

This application claims benefit under 35 USC 119 of ProvisionalApplication 60/882,081 filed Dec. 27, 2006.

FIELD OF THE INVENTION

The present invention relates to the treatment of cold start engineexhaust using certain zeolites having different pore sizes.

BACKGROUND

Gaseous waste products resulting from the combustion ofhydrocarbonaceous fuels, such as gasoline and fuel oils, comprise carbonmonoxide, hydrocarbons and nitrogen oxides as products of combustion orincomplete combustion, and pose a serious, health problem with respectto pollution of the atmosphere. While exhaust gases from othercarbonaceous fuel-burning sources, such as stationary engines,industrial furnaces, etc., contribute substantially to air pollution,the exhaust gases from automotive engines are a principal source ofpollution. Because of these health problem concerns, the EnvironmentalProtection Agency (EPA) has promulgated strict controls on the amountsof carbon monoxide, hydrocarbons and nitrogen oxides which automobilescan emit. The implementation of these controls has resulted in the useof catalytic converters to reduce the amount of pollutants emitted fromautomobiles.

In order to achieve the simultaneous conversion of carbon monoxide,hydrocarbon and nitrogen oxide pollutants, it has become the practice toemploy catalysts in conjunction with air-to-fuel ratio control meanswhich functions in response to a feedback signal from an oxygen sensorin the engine exhaust system. Although these three component controlcatalysts work quite well after they have reached operating temperatureof about 300° C., at lower temperatures they are not able to convertsubstantial amounts of the pollutants. What this means is that when anengine and in particular an automobile engine is started up, the threecomponent control catalyst is not able to convert the hydrocarbons andother pollutants to innocuous compounds.

Adsorbent beds have been used to adsorb the hydrocarbons during the coldstart portion of the engine. Although the process typically wilt be usedwith hydrocarbon fuels, adsorbent beds can also be used to treat exhauststreams from alcohol fueled engines. The adsorbent bed is typicallyplaced immediately before the catalyst. Thus, the exhaust stream isfirst flowed through the adsorbent bed and then through the catalyst.The adsorbent bed preferentially adsorbs hydrocarbons over water underthe conditions present in the exhaust stream. After a certain amount oftime, the adsorbent bed has reached a temperature (typically about 150°C.) at which the bed is no longer able to remove hydrocarbons from theexhaust stream. That is, hydrocarbons are actually desorbed from theadsorbent bed instead of being adsorbed. This regenerates the adsorbentbed so that it can adsorb hydrocarbons during a subsequent cold start.

The prior art reveals several references dealing with the use ofadsorbent beds to minimize hydrocarbon emissions during a cold startengine operation. One such reference is U.S. Pat. No. 3,699,683 in whichan adsorbent bed is placed after both a reducing catalyst and anoxidizing catalyst. The patentees disclose that when the exhaust gasstream is below 200° C. the gas stream is flowed through the reducingcatalyst then through the oxidizing catalyst and finally through theadsorbent bed, thereby adsorbing hydrocarbons on the adsorbent bed. Whenthe temperature goes above 200° C. the gas stream which is dischargedfrom the oxidation catalyst is divided into a major and minor portionthe major portion being discharged directly into the atmosphere and theminor portion passing through the adsorbent bed whereby unburnedhydrocarbon is desorbed and then flowing the resulting minor portion ofthis exhaust stream containing the desorbed unburned hydrocarbons intothe engine where they are burned.

Another reference is U.S. Pat. No. 2,942,932 which teaches a process foroxidizing carbon monoxide and hydrocarbons which are contained inexhaust gas streams. The process disclosed in this patent consists offlowing an exhaust stream which is below 800° F. into an adsorption zonewhich adsorbs the carbon monoxide and hydrocarbons and then passing theresultant stream from this adsorption zone into an oxidation zone. Whenthe temperature of the exhaust gas stream reaches about 800° F. theexhaust stream is no longer passed through the adsorption zone but ispassed directly to the oxidation zone with the addition of excess air.

U.S. Pat. No. 5,078,979, issued Jan. 7, 1992 to Dunne, which isincorporated herein by references in its entirety, discloses treating anexhaust gas stream from an engine to prevent cold start emissions usinga molecular sieve adsorbent bed. Examples of the molecular sieve includefaujasites clinoptilolites, mordenites, chabazite, silicalite, zeoliteY, ultrastable zeolite Y, and ZSM-5.

Canadian Patent No. 1,205,980 discloses a method of reducing exhaustemissions from an alcohol fueled automotive vehicle. This methodconsists of directing the cool engine startup exhaust gas through a bedof zeolite particles and then over an oxidation catalyst and then thegas is discharged to the atmosphere. As the exhaust gas stream warms upit is continuously passed over the adsorption bed and then over theoxidation bed.

U.S. Pat. No. 5,744,103, issued Apr. 28, 1998 to Yamada et al.,discloses a hydrocarbon adsorbent for engine exhaust gas cleaning. Theadsorbent contains large pore zeolites having 12+ membered rings (“MR”),smaller pore zeolites having 8 MR and in-between pore zeolites having 10MR. Disclosed examples of the zeolites are those having the topologies(as identified by the International Zeolite Association (“IZA”)) FAU(e.g., zeolite Y), AFY and Beta (i.e., 12 MR zeolites); CHA (8 MR); andMFI (e.g., ZSM-5), MEL and FER (10 MR).

U.S. Pat. No. 5,603,216, issued Feb. 18, 1997 to Guile et al., disclosesreducing the amount of hydrocarbons emitted during engine start (coldstart) using two zones in the exhaust system using the same or differentzeolite adsorber(s) in each zone. The zeolite(s) may be small porezeolite which adsorbs low molecular weight alkenes (ethylene andpropylene) and large pore zeolite which adsorb higher molecular weighthydrocarbons (e.g., pentane). Disclosed examples of zeolites are ZSM-5,Beta, gmelinite, mazzite, offretite, ZSM-12, ZSM-18,Berryllophosphate-H, boggsit, SAPO-40, SAPO-41, Ultrastable Y, mordeniteand combinations thereof.

Elangovan et al. Journal of Physical Chemistry B, 108, 13059-13061(2004) discloses the zeolite designated SSZ-33 (a zeolite havingintersecting 10 and 12 MR pores with a large void at the intersections)for use as a hydrocarbon trap to reduce cold start emissions. Theperformance of the SSZ-33 is compared to that of Beta, Y, mordenites andZSM-5 zeolites. SSZ-33 is said to have superior performance over Beta,Y, mordenites or ZSM-5.

U.S. Patent Application Publication 2005/0166581, published Aug. 4, 2005by Davis et al., discloses molecular sieves used as adsorbents inhydrocarbon traps for engine exhaust. The method comprises contactingthe exhaust gas with molecular sieves having the CON topology (per theIZA). The CON molecular sieve can be used by itself or can be used withanother adsorbent. Disclosed examples of CON molecular sieves are thosedesignated SSZ-33, SSZ-26, and CIT-1. ITQ-4 is also disclosed, but it isbelieved ITQ-4 has the IFR topology, not the CON topology. Disclosedexamples of the other adsorbent are molecular sieves designated SSZ-23,SSZ-31, SSZ-35, SSZ-44, SSZ-42, SSZ-43, SSZ-44, SSZ-45, SSZ-47, SSZ-48,SSZ-53, SSZ-55, SSZ-57, SSZ-58, SSZ-59, SSZ-60, SSZ-63, SSZ-64, SSZ-65and mixtures thereof.

SUMMARY OF THE INVENTION

This invention generally relates to a process for treating an engineexhaust stream and in particular to a process for minimizing emissionsduring the cold start operation of an engine. Accordingly, the presentinvention provides a process for treating a cold-start engine exhaustgas stream containing hydrocarbons and other pollutants consisting offlowing said engine exhaust gas stream over a combination of molecularsieves which preferentially adsorbs the hydrocarbons over water toprovide a first exhaust stream, and flowing the first exhaust gas streamover a catalyst to convert any residual hydrocarbons and otherpollutants contained in the first exhaust gas stream to innocuousproducts and provide a treated exhaust stream and discharging thetreated exhaust stream into the atmosphere, the combination of molecularsieves comprising (1) a small pore crystalline molecular sieve ormixture of molecular sieves having pores no larger than 8 membered rings(“8 MR”) selected from the group consisting of SSZ-13, SSZ-16, SSZ-36,SSZ-39, SSZ-50, SSZ-52 and SSZ-73 and having a mole ratio of at least 13of (a) an oxide of a first tetravalent element to (b) an oxide of atrivalent element pentavalent element, second tetravalent element whichis different from said first tetravalent element or mixture thereof and(2) a medium-large pore crystalline molecular sieve having pores atleast as large as 10 membered rings (“10 MR”) selected from the groupconsisting of SSZ-26, SSZ-33, SSZ-64, zeolite Beta, CIT-1, CIT-6 andITQ-4 and having a mole ratio of at least 10 of (a) an oxide of a firsttetravalent element to (b) an oxide of a trivalent element, pentavalentelement, second tetravalent element which is different from said firsttetravalent element or mixture thereof. The present invention alsoprovides such a process wherein oxides (1)(a) and (2)(a) are siliconoxide, and oxides (1)(b) and (2)(b) are independently selected fromaluminum oxide, gallium oxide, iron oxide, boron oxide, titanium oxide,indium oxide, zinc oxide, magnesium oxide, cobalt oxide and mixturesthereof. In one embodiment, molecular sieve (1) is SSZ-13, SSZ-39 ormixtures thereof and molecular sieve (2) is SSZ-26, SSZ-33, CIT-1, BetaCIT-6 or mixtures thereof. In another embodiment, molecular sieve (1),molecular sieve (2) or both contain a metal selected from Cu, Ag, Au ormixtures thereof.

The present invention further provides such a process wherein the engineis an internal combustion engine, including automobile engines which canbe fueled by a hydrocarbonaceous fuel.

Also provided by the present invention is such a process wherein themolecular sieve has deposited on it a metal selected from the groupconsisting of platinum, palladium, rhodium, ruthenium, and mixturesthereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-3 illustrate data comparing the adsorptive properties ofzeolites with the adsorptive properties of mixtures of zeolitesaccording to the present invention.

FIG. 1 illustrates zeolite methane adsorption at 30° C.

FIG. 2 illustrates zeolite ethane adsorption at 30° C.

FIG. 3 illustrates zeolite ethylene adsorption at 30° C.

DETAILED DESCRIPTION OF THE INVENTION

As stated, this invention generally relates to a process for treating anengine exhaust stream and in particular to a process for minimizingemissions during the cold start operation of an engine. The engineconsists of any internal or external combustion engine which generatesan exhaust gas stream containing noxious components or pollutantsincluding unburned or thermally degraded hydrocarbons or similarorganics. Other noxious components usually present in the exhaust gasinclude nitrogen oxides and carbon monoxide. The engine may be fueled bya hydrocarbonaceous fuel. As used in this specification and in theappended claims, the term “hydrocarbonaceous fuel” includeshydrocarbons, alcohols and mixtures thereof. Examples of hydrocarbonswhich can be used to fuel the engine are the mixtures of hydrocarbonswhich make up gasoline or diesel fuel. The alcohols which may be used tofuel engines include ethanol and methanol. Mixtures of alcohols andmixtures of alcohols and hydrocarbons can also be used. The engine maybe a jet engine, gas turbine; internal combustion engine, such as anautomobile, truck or bus engine; a diesel engine or the like. Theprocess is particularly suited for hydrocarbon; alcohol, orhydrocarbon-alcohol mixture, internal combustion engine mounted in anautomobile. For convenience the description will use hydrocarbon as thefuel to exemplify the invention. The use of hydrocarbon in thesubsequent description is not to be construed as limiting the inventionto hydrocarbon fueled engines.

When the engine is started up, it produces a relatively highconcentration of hydrocarbons in the engine exhaust gas stream as wellas other pollutants. Pollutants will be used herein to collectivelyrefer to any unburned fuel components and combustion by products foundin the exhaust stream. For example, when the fuel is a hydrocarbon fuel,hydrocarbons, nitrogen oxides, carbon monoxide and other combustionbyproducts will be found in the engine exhaust gas stream. Thetemperature of this engine exhaust stream is relatively cool, generallybelow 500° C. and typically in the range of 200 to 400° C. This engineexhaust stream has the above characteristics during the initial periodof engine operation, typically for the first 30 to 120 seconds afterstartup of a cold engine. The engine exhaust stream will typicallycontain, by volume, about 500 to 1000 ppm hydrocarbons.

The engine exhaust gas stream which is to be treated is flowed over acombination of molecular sieves in a first exhaust stream. Thecombination of molecular sieves is described below. The first exhauststream which is discharged from the molecular sieve combination is nowflowed over a catalyst to convert the pollutants contained in the firstexhaust stream to innocuous components and provide a treated exhauststream which is discharged into the atmosphere. It is understood thatprior to discharge into the atmosphere, the treated exhaust stream maybe flowed through a muffler or other sound reduction apparatus wellknown in the art.

The catalyst which is used to convert the pollutants to innocuouscomponents is usually referred to in the art as a three-componentcontrol catalyst because it can simultaneously oxidize any residualhydrocarbons present in the first exhaust stream to carbon dioxide andwater, oxidize any residual carbon monoxide to carbon dioxide and reduceany residual nitric oxide to nitrogen and oxygen. In some cases thecatalyst may not be required to convert nitric oxide to nitrogen andoxygen, e.g., when an alcohol is used as the fuel. In this case thecatalyst is called an oxidation catalyst. Because of the relatively lowtemperature of the engine exhaust stream and the first exhaust stream,this catalyst does not function at a very high efficiency therebynecessitating the molecular sieve adsorbent.

When the molecular sieve adsorbent reaches a sufficient temperature,typically about 150-200° C., the pollutants which are adsorbed in themolecular sieve(s) begin to desorb and are carried by the first exhauststream over the catalyst. At this point the catalyst has reached itsoperating temperature and is therefore capable of fully converting thepollutants to innocuous components.

The adsorption capacity of a molecular sieve depends on the size of thehydrocarbon molecule (thus, its molecular weight and shape). Forexample, when a molecular sieve having a smaller pore diameter (such asan eight MR pore) is used, the hydrocarbons having higher molecularweights (such as paraffins, olefins or aromatic compounds having atleast six carbon atoms) may not be adsorbed. To the contrary, when amolecular sieve having a medium-large pore opening diameter (such as atwelve and/or ten MR pore) is, used, hydrocarbons having lower molecularweights (such as methane, propane or propylene) is desorbed at a lowerthan desired temperature, so that it is difficult to keep such ahydrocarbon in the pores of the medium-large pore molecular sieve untilthe noble metal becomes of sufficiently high temperature to beactivated.

The molecular sieve adsorbent used in the present invention comprises acombination or mixture of molecular sieves containing (1) a molecularsieve or mixture of molecular sieves having pores no larger than 8membered rings (“8 MR”) selected from the group consisting of SSZ-13,SSZ-16, SSZ-36, SSZ-39, SSZ-50, SSZ-52 and SSZ-73 and having a moleratio of at least 10 of (a) an oxide of a first tetravalent element to(b) an oxide of a trivalent element, pentavalent element, secondtetravalent element which is different from said first tetravalent:element or mixture thereof and (2) a medium-large pore crystallinemolecular sieve having pores at least as large as 10 membered rings (“10MR”) selected from the group consisting of SSZ-26, SSZ-33, SSZ-64,zeolite Beta, CIT-1, CIT-6 and ITQ-4 and having a mole ratio of at least10 of (a) an oxide of a first tetravalent element to (b) an oxide of atrivalent element, pentavalent element, second tetravalent element whichis different from said first tetravalent element or mixture thereof.

The small pore molecular sieves of this invention have (2-dimensional or3-dimensional) intersecting channels. Examples of such molecular sievesinclude the following, where the three letter structure code, number ofmembers in the pore ring(s) and channel configuration are from theInternational Zeolite Association database.

Molecular sieve designated SSZ-13 (CHA) disclosed in U.S. Pat. No.4,544,538, issued Oct. 1, 1985 to Zones.

Molecular sieve designated SSZ-16 (AFX, 8 6 4), 3D) disclosed in U.S.Pat. No. 5,958,370, issued Sep. 28, 1999 to Zones et al.

Molecular sieve designated SSZ-36 (ITE-RTH structural intermediate),disclosed in U.S. Pat. No. 5,939,044, issued Aug. 17, 1999 to Nakagawaet al.

Molecular sieve designated SSZ-39 (AEI, (8 6 4), 3D), disclosed in U.S.Pat. No. 5,958,370, issued Sep. 28, 1999.

Molecular sieve designated SSZ-50 (RTH, (8 6 5 4), 2D), disclosed inU.S. Pat. No. 6,605,267, issued Aug. 12, 2003.

Molecular sieve designated SSZ-52, disclosed in U.S. Pat. No. 6,254,849,issued Jul. 3, 2001.

Molecular sieve designated SSZ-73, disclosed in U.S. Pat. No. 7,138,099,issued Nov. 21, 2006.

The aforementioned patents cited to identify the small pore molecularsieves useful in this invention are incorporated herein by reference intheir entirety.

The small pore molecular sieves listed above fulfill the criteria foruse in the present invention of having large: micropore volumes and highratios of oxide(s) (1), e.g., silica to oxide(s) (2), e.g., alumina(referred to herein as “high silica” molecular sieves). These twofeatures distinguish these small pore molecular sieves from molecularsieves used in the prior art wherein the small pore molecular sievescontained large aluminum contents. This latter feature renders them muchmore sensitive to collapse (sensitivity to steam under operatingconditions) than the high silica, small pore molecular sieves of thisinvention.

The medium-large pore molecular sieves useful in this invention have(2-dimensional or 3-dimensional) intersecting channels. The medium-largepore molecular sieves should; have a high internal pore volume (e.g., anitrogen adsorption capacity of about 0.18 cc/gm or higher). Examples ofsuch molecular sieves include the following, where the three letterstructure code, number of members in the pore ring(s) and channelconfiguration are from the International Zeolite Association database:

Molecular sieve designated SSZ-26 (CON), disclosed in U.S. Pat. No.4,910,006, issued Mar. 20, 1990 to Zones et al.

Molecular sieve designated SSZ-33 (CON), disclosed in U.S. Pat. No.4,963,337, issued Oct. 16, 1990 to Zones.

Molecular sieve designated SSZ-64, disclosed is U.S. Pat. No. 6,569,401,issued May 27, 2003 to Elomari. SSZ-64 is believed to have a disorderedstructure with at least one 12 MR in the structure and a microporevolume that exceeds 0.20 cc/g.

Molecular sieve designated zeolite Beta (*BEA), disclosed in U.S. Pat.No. 3,308,069, issued Mar. 7, 1967 to Wadlinger et al., and Re. 28,341,reissued Feb. 18, 1975 to Wadrlinger et al.

Molecular sieve designated CIT-1 (CON), disclosed in U.S. Pat. No.5,512,267, issued Apr. 30, 1996 to Davis et al.

Molecular sieve designated CIT-6 (*BEA), disclosed in U.S. Pat. No.6,117,411, issued Sep. 12, 2000 to Takewaki et al.

Molecular sieve designated ITQ-24 (IWR), disclosed in R. Castaneda etal., Synthesis of a New Zeolite Structure ITQ-24 with Interesting 10-and 12-Membered Ring Pores, J. Am. Chem. Soc., 125, 7820-7821 (203).

The aforementioned patents and literature article identifying themedium-large pore molecular sieves used in the present invention areincorporated herein by reference in their entirety.

The molecular sieves may comprise a framework heteroatom such as Al, B,Ga, Fe, Zn, Mg, Co and mixtures thereof in addition to Si. The molecularsieves may also contain a metal cation selected from rare earth, Group 2metals, Groups 8-10 metals and mixtures thereof, e.g., the metal cationmay be selected from Mn, Ca, Mg, Zn, Cd, Pt, Pd, Ni, Co, Ti, Al, Sn, Fe,Co and mixtures thereof. The molecular sieves may also contain a metalselected from Cu, Ag, Au and mixtures thereof. The molecular sieves mayalso include other partial replacement atoms for Si such as Ge.Techniques for replacing Si with Ge are known in the art (see, forexample, U.S. Pat. Nos. 4,910,006 and 4,963,337).

In one embodiment, the combination of small and medium-large poremolecular sieves is a combination of (1) SSZ-13 and (2) SSZ-26, SSZ-33or mixtures thereof. In another embodiment, the combination of small andmedium-large pore molecular sieves is a combination of (1) SSZ-13 and(2) SSZ-26.

The molecular sieves should be thermally stable to about 700° C., suchas in the presence of steam. Steam can remove some metals, such asaluminum from the framework of some zeolites, causing their structure tocollapse. Thus, it is important that the molecular sieves used in thepresent invention be steam stable. If the molecular sieve to be useddoes contain a metal, such as zinc, in the framework which makes themolecular sieve unstable in a steam environment, that metal can bereplaced with an element that makes the molecular sieve steam stable.

If the hydrocarbonaceous fuel undergoes incomplete combustion in theengine, the exhaust gas can contain carbon dioxide and water. Thepresence of water in the exhaust gas can make some molecular seizesunstable. One way of stabilizing such molecular sieves is to increasethe amount of silicon oxide in the molecular sieve. In general, thehigher the silicon oxide content, the more hydrophobic the molecularsieve will be, and the more stable it will be in the present of watervapor. Thus, it may be desirable to partially or completely replace somemetals (such as zinc) in the framework of the molecular sieve withsilicon to increase hydrophobicity. In some cases, molecular sievescontaining all-silicon oxide may be desirable.

The small pore molecular sieve(s) and the medium-large pore molecularsieve(s) of this invention are used in combination to treat a cold-startengine exhaust gas stream. As used herein, the term “combination” meansthat the cold-start engine exhaust gas stream is contacted with both thesmall pore molecular sieve(s) of this invention and the medium-largepore molecular sieve(s) of this invention prior to the exhaust streamentering the catalytic converter. This can be accomplished in a numberof ways. For example, the “combination” may comprise a mixture of thesmall pore and medium-large pore molecular sieves in, e.g., a singlebed. The small and large pore molecular sieves may also be used inseparated beds, or in a single bed comprising layers of small pore andmedium-large pore molecular sieves. The small and medium-large poremolecular sieves may also be used in a single bed in which theconcentration of one, e.g., the small pore molecular sieve(s), is highand the concentration of the medium-large pore molecular sieve(s) is low(possibly as low as zero) at the upstream side of the bed. Theconcentration of the small and medium-large pore molecular sieves thengradually reverses in the downstream direction such that theconcentration of the, e.g., small pore molecular sieve(s) is low(possibly as low as zero) and the concentration of the, e.g.,medium-large pore molecular sieve(s) is high at the downstream end ofthe bed. However, the small and medium-large pore molecular sieves areconveniently disposed in separate, discrete beds. When used as such, itis possible that, in the event one of the beds fouls, only the fouledbed need be replaced, leaving the other bed intact.

The order in which the cold-start exhaust gas contacts the small poremolecular(s) and the medium-large pore molecular sieve(s) may not becritical. However, there may be advantages to contacting the cold startexhaust gas with the small pore molecular sieve(s) prior to contact withthe medium-large pore molecular sieve(s). In this configuration, smallerhydrocarbons (e.g., methane, propane and/or propylene) can be adsorbedby the small pore molecular sieves, While the larger hydrocarbons bypassthe small pore molecular sieve(s) (because they are too large to fit inthe small pores) leaving the medium-large pore molecular sieve(s) freeto adsorb the larger hydrocarbons. The opposite configuration (i.e.,medium-large pore molecular sieve(s) positioned upstream of the smallpore molecular sieve(s)) may be used as well. However, in this case,there is a risk that the smaller hydrocarbons will fill the pores of themedium-large pore molecular sieve(s) and block entry of the largerhydrocarbons. In that event, the larger hydrocarbons may bypass thepore-filled medium-large pore molecular sieve(s) as well as thedownstream small pore molecular sieve(s) (which are incapable ofadsorbing the larger hydrocarbons) and proceed to the catalyticconverter before the catalytic converter's temperature has risen to atemperature sufficient to convert the larger hydrocarbons.

The particular configuration of the combination can take many forms. Forinstance, the adsorbent bed can be conveniently employed in particulateform or the adsorbent can be deposited onto a solid monolithic carrier.When the particulate form is desired, the adsorbent can be used in theform of powders, pills, pellets, granules, rings, spheres, etc. In theemployment of a monolithic form, it is usually most convenient to employthe adsorbent as a thin film or coating deposited on an inert carriermaterial which provides the structural support for the adsorbent. Theinert carrier material can be any refractory material such as ceramic ormetallic materials. It is desirable that the carrier material beunreactive with the adsorbent and not be degraded by the gas to which itis exposed. Examples of suitable ceramic materials include sillimanite,petalite, cordierte, mullite, zircon, zircon mullite, spondumene,alumina-titanate, etc. Examples of metallic materials which serve asinert carrier material include metals and alloys as disclosed in U.S.Pat. No. 3,920,583 which are oxidation resistant and are otherwisecapable of withstanding high temperatures.

The carrier material can be utilized in any rigid unitary configurationwhich provides a plurality of pores or channels extending in thedirection of gas flow. Conveniently, the configuration may be ahoneycomb configuration. The honeycomb structure can be usedadvantageously in either unitary form, or as an arrangement of multiplemodules. The honeycomb structure is usually oriented such that gas flowis generally in the same direction as the cells or channels of thehoneycomb structure. For a more detailed discussion of monolithicstructures, refer to U.S. Pat. Nos. 3,785,998 and 3,767,453 which areincorporated by reference herein.

The molecular sieve combination can be deposited onto the carrier by anyconvenient way well known in the art. One convenient method involvespreparing a slurry using the molecular sieves which form the combination(either together in a single slurry or separately in different slurries)and coating the monolithic honeycomb carrier with the slurry(ies). Theslurry(ies) can be prepared by means known in the art such as combiningthe appropriate amount of the molecular sieve(s) and a binder withwater. This resulting mixture(s) is then blended by using means such assonification, milling, etc. This slurry(ies) is used to coat amonolithic honeycomb by dipping the honeycomb into the slurry(ies),removing the excess slurry(ies) by draining or blowing out the channels,and heating to about 100° C. If the desired loading of molecular sievecombination is not achieved, the above process may be repeated as manytimes as required to achieve the desired loading.

Instead of depositing the molecular sieve combination onto a monolithichoneycomb structure, one can take the molecular sieve combination andform it into a monolithic honeycomb structure by means known in the art.

The adsorbent may optionally contain one or more catalytic metalsdispersed thereon. The metals which can be dispersed on the adsorbentare the noble metals which consist of platinum, palladium, rhodium,ruthenium, and mixtures thereof. The desired noble metal may bedeposited onto the adsorbent, which acts as a support, in any suitablemanner well known in the art. One example of a method of dispersing thenoble metal onto the adsorbent sup) port involves impregnating theadsorbent support with an aqueous solution of a decomposable compound ofthe desired noble metal or metals, drying the adsorbent which has thenoble metal compound dispersed on it and then calcining. In air at atemperature of about 400° to about 500° C. for a time of about 1 toabout 4 hours. By decomposable compound is meant a compound which uponheating in air gives the metal or metal oxide. Examples of thedecomposable compounds which can be used are set forth in U.S. Pat. No.4,791,091 which is incorporated by reference. Preferred decomposablecompounds are chloroplatinic acid, rhodium trichloride, chloropalladicacid, hexachloroiridate (IV) acid and hexachlororutahenate. It ispreferable that the noble metal be present in an amount ranging fromabout 0.01 to about 4 weight percent of the adsorbent support.Specifically, in the case of platinum and palladium the range is 0.1 to4 weight percent, while in the case of rhodium and, ruthenium the rangeis from about 0.01 to 2 weight percent.

These catalytic metals are capable of oxidizing the hydrocarbon andcarbon monoxide and reducing the nitric oxide components to innocuousproducts. Accordingly, the adsorbent bed can act both as an adsorbentand as a catalyst.

The catalyst in the catalytic converter may be selected from any threecomponent control or oxidation catalyst well known in the art. Examplesof catalysts are those described in U.S. Pat. Nos. 4,528,279, 4,791,091;4,760,044, 4,868,148; and 4,868,149, which are all incorporated byreference. Preferred catalysts well known in the art are those thatcontain platinum and rhodium and optionally palladium, while oxidationcatalysts usually do not contain rhodium. Oxidation catalysts usuallycontain platinum and/or palladium metal. These catalysts may alsocontain promoters and stabilizers such as barium, cerium, lanthanum,nickel, and iron. The noble metals promoters and stabilizers are usuallydeposited on a support such as alumina, silica, titania, zirconia,alumino silicates, and mixtures thereof with alumina being preferred.The catalyst can be conveniently employed in particulate form or thecatalytic composite can be deposited on a solid monolithic carrier witha monolithic carrier being preferred. The particulate form andmonolithic form of the catalyst are prepared as described for theadsorbent above.

EXAMPLE 1

The adsorption characteristics of zeolites were tested for theireffectiveness for adsorbing hydrocarbon materials normally found inautomobile exhaust streams. The SSZ-13 sample used in these tests had asilica/alumina ratio of 15.6. The SSZ-33 sample used in these tests hada silica/alumina ratio of 14.6.

Approximately 70 milligrams of a sample of zeolite SSZ-13 powder wereloaded in turn into a VTI Scientific Instruments GHP-FS GravimetricSorption Analyzer. The sample preparation consisted of drying the sampleat 350° C. for 300 minutes (or until the sample weight changed by lessthan 0.005% over a ten-minute period). The sample then was allowed toequilibrate with methane gas at 30° C. and 500 torr pressure for 30minutes, or until the sample weight changed by less than 0.005% over afifteen-minute interval. The pressure was then increased at 500 torrintervals up to a maximum of 5000 torr, with the sample being allowed toequilibrate with the methane gas after each pressure increase. At eachpressure, the methane adsorption amount was determined by the weightchange of the sample.

The method was repeated using a: sample of zeolite SSZ-33 powder.

The method was repeated using a physical mixture of zeolite SSZ-13powder and zeolite SSZ-33 powder.

The methane uptake by the following zeolites and uniform physicalmixtures of zeolites, reported in mmole/gram, is plotted in FIG. 1.

Sample ID Zeolite SSZ13 100% SSZ-13 0.8SSZ13 4:1 weight ratioSSZ-13:SSZ-33 0.75SSZ13 3:1 weight ratio SSZ-13:SSZ-33 0.5SSZ13 1:1weight ratio SSZ-13:SSZ-33 SSZ33 100% SSZ-33

The data illustrated in FIG. 1 shows that methane uptake by SSZ-13 at30° C. was greater on a weight basis than that of SSZ-33. The 1:1mixture of SSZ-13: SSZ-33 (denoted 0.5SSZ13 in FIG. 1) was intermediatebetween the methane uptake of the two zeolites alone. However, the 4:1and 3:1 mixtures of SSZ-13: SSZ-33 (denoted 0.8 SSZ-13 and 0.75SSZ13,respectively, in FIG. 1) are shown to have a methane uptake which wasequal to, or greater than that of SSZ-13 and SSZ-33 alone.

EXAMPLE 2

Example 1 was repeated using ethane as an adsorbent. The ethane uptakeby the following zeolite samples, reported in mmole/gram, is plotted inFIG. 2.

Sample ID Zeolite SSZ13 100% SSZ-13 0.5SSZ13 1:1 weight ratioSSZ-13:SSZ-33 SSZ33 100% SSZ-33

The data illustrated in FIG. 2 shows that the 1:1 mixture of SSZ13:SSZ33(denoted 0.5SSZ13) had an ethane uptake at 30° C. which was intermediatebetween that of SSZ-13 and SSZ-33 alone.

EXAMPLE 3

Example 1 was repeated using ethylene as an adsorbent. The ethyleneuptake by the following zeolite samples, reported in mmole/gram, isplotted in FIG. 3.

Sample ID Zeolite SSZ13 100% SSZ-13 0.75SSZ13 3:1 weight ratioSSZ-13:SSZ-33 0.5SSZ13 1:1 weight ratio SSZ-13:SSZ-33 SSZ33 100% SSZ-33

The data illustrated in FIG. 3 show that the 3:1 and 1:1 mixtures ofSSZ-13 and SSZ-33 had a much higher ethylene uptake at 30° C. than thatof SSZ-13 and SSZ-33 alone.

1. A process for treating a cold-start engine exhaust gas streamcontaining hydrocarbons and, other pollutants consisting of flowing saidengine exhaust gas stream over a combination of molecular sieves whichpreferentially adsorbs the hydrocarbons over water to provide a firstexhaust stream, and flowing the first exhaust gas stream over a catalystto convert any residual hydrocarbons and other pollutants contained inthe first exhaust gas stream to innocuous products and provide a treatedexhaust stream and discharging the treated exhaust stream into theatmosphere, the combination of molecular sieves comprising (1) a smallpore crystalline molecular sieve or mixture of molecular sieves havingpores no larger than 8 membered rings selected from the group consistingof SSZ-13, SSZ-16, SSZ-36, SSZ-39, SSZ-50, SSZ-52 and SSZ-73 molecularsieve and having a mole ratio of at least 10 of (a) an oxide of a first:tetravalent element to (b) an oxide of a trivalent element, pentavalentelements, second tetravalent element which is different from said firsttetravalent element or mixture thereof and (2) a medium-large porecrystalline molecular sieve having pores at least as large as 10membered rings selected from the group consisting of SSZ-26, SSZ-33,SSZ-64 zeolite Beta, CIT-1, CIT-6 and ITQ-4 and having a mole ratio ofat least 10 of (a) an oxide of a first tetravalent element to (h) anoxide of a trivalent element, pentavalent element, second tetravalentelement which is different from said first tetravalent element ormixture thereof.
 2. The process of claim 1 wherein oxides (1)(a) and(2)(a) are silicon oxide, and oxides (1)(b) and (2)(b) are independentlyselected from aluminum oxide, gallium oxide, iron oxide, boron oxide,titanium oxide, indium oxide, zinc, oxide, magnesium oxide, cobalt oxideand mixtures thereof.
 3. The process of claim 2 wherein oxides (1)(b)and (2)(b) are aluminum oxide.
 4. The process of claim 2 whereinmolecular sieve (1) is SSZ-73, SSZ-39 or mixtures thereof and molecularsieve (2) is SSZ-26, SSZ-33, CIT-1, Beta, CIT-6 or mixtures thereof. 5.The process of claim 4 wherein molecular sieve (1) is SSZ-13 andmolecular sieve (2) is SSZ-26, SSZ-33 or mixtures thereof.
 6. Theprocess of claim 5 wherein molecular sieve (2) is SSZ-26.
 7. The processof claim 2 wherein molecular sieve (1), molecular sieve (2) or bothcontain a metal selected from Cu, Ag, Au or mixtures thereof.
 8. Theprocess of claim 2 wherein the engine is an internal combustion engine.9. The process of claim 8 wherein the terminal combustion engine is anautomobile engine.
 10. The process of claim 1 or 2 wherein the engine isfueled by a hydrocarbonaceous fuel.
 11. The process of claim 1 or 2wherein the molecular sieve has deposited on it a metal selected fromthe group consisting of platinum palladium, rhodium, ruthenium, andmixtures thereof.
 12. The process of claim 11 wherein the metal isplatinum.
 13. The process of claim 11 wherein the metal is palladium.14. The process of claim 11 wherein the metal is a mixture of platinumand palladium.
 15. A process for removing methane from a gaseoushydrocarbon stream comprising passing a methane-containing hydrocarbonstream over an adsorbent comprising a small pore crystalline molecularsieve and a medium-large pore crystalline molecular sieve at atemperature in the range of 10 C to 75 C and recovering a product streamwith reduced methane content, wherein the weight ratio of the molecularsieves in the adsorbent is in the weight range of 5:1 to 2:1 of smallpore/medium-large pore crystalline molecular sieve.
 16. The profess ofclaim 15 wherein the small pore crystalline molecular sieve is SSZ-13and the medium-large pore crystalline molecular sieve is SSZ-33.
 17. Aprocess for removing ethylene from a gaseous hydrocarbon streamcomprising passing a ethylene-containing hydrocarbon stream over anadsorbent comprising a small pore crystalline molecular sieve and amedium-large pore crystalline molecular sieve at a temperature in therange of 10 C to 75 C and recovering a product stream with reducedethylene content, wherein the weight ratio of the molecular sieves inthe adsorbent is in the weight range of 5:1 to 1:2 of smallpore/medium-large pore crystalline molecular sieve.
 18. The process ofclaim 17 wherein the small pore crystalline molecular sieve is SSZ-13and the medium-large pore crystalline molecular sieve is SSZ-33.
 19. Theprocess of claim 17 wherein the weight ratio of the molecular sieves inthe adsorbent is in the weight range of 5:1 to 2:1 a of smallpore/medium-large pore crystalline molecular sieve.